Detectors for diagnostic imaging systems, for example, detectors for single photon emission computed tomography (SPECT) and computed tomography (CT) imaging systems are often produced from semiconductor materials, such as Cadmium Zinc Telluride (CdZnTe), often referred to as CZT, Cadmium Telluride (CdTe) and Silicon (Si), among others. These semiconductor detectors typically include arrays of pixelated detector modules.
Ideally, photon absorption (an event) occurs in a single pixel. However, in pixelated detectors, charge-sharing between two or more adjacent pixels may occur. Charge-sharing is caused by photon absorption in a gap between adjacent pixels. Charge-sharing events cause each of the associated signals for the pixels to be out of a photo-absorption energy window, and, therefore, rejected as being generated by photons that suffer from Compton Scattering in the body of a patient, which are not suitable for imaging purposes.
Also, Compton Scattering may occur in the detector, with the amount of Compton Scattering inside the detector increasing with photon energy. Photons absorbed in the detector may be absorbed by one step including photo-electric absorption or by a series of multiple steps including one or more steps of Compton Scattering that ends with photo-electric absorption. Compton Scattering may occur in several adjacent pixels. In such a case, the energy of the photon is absorbed and shared between several pixels, causing each of the signals for the pixels to be out of the photo-absorption energy window. Such signals may thus be interpreted as being generated by photons that suffer Compton Scattering in the patient body and rejected.
Neighbor summing (summing signals received in adjacent pixels to recover the energy of neighboring pixels into one signal that is located within the photo-electric absorption energy window) may be attempted to address these issues. Conventionally, neighbor summing may be performed by a variety of techniques including verifying if the signals are in time coincidence within a specified time window.
However, such neighbor summing is not without drawbacks. For example, the determination or verification of whether signals are in time coincidence may present drawbacks. In CZT detectors, for example, the timing of a trigger signal indicating timing proportional to the absorption time of a photon in a pixel depends on the depth of interaction (DOI) of the absorbed photon in the detector. Thus, the timing of the trigger signal is strongly dependent on the DOI and therefore is not accurate enough to serve for time coincident measurements. Accordingly, the trigger signal may be derived from the cathode of the CZT detector. The cathode is a large contact and may produce a trigger signal immediately upon absorption of a photon in the detector. However, deriving the trigger signal from the cathode is difficult to implement because the signal is noisy. Also, the signal may need to be fed into an input from a remote distance. The relatively high noise produced by a large cathode requires the use of a relatively high threshold level in the input of a comparator in an electronic channel of a pixel to prevent propagation, in the electronic channels of the detector pixels, of the relatively high noise produced by the large cathode. The use of the high threshold level also causes rejection of all signals below the relatively high threshold level. Thus, many events for which charges are shared between pixels may be rejected as being too small due to the required high threshold level when the signal is derived from the cathode. As a result, the summing process may be inefficient when using timing derived from the cathode contact. Additionally, adding timing circuitry to the camera's hardware may require modification of the front end electronics. This may add to the complexity, price and energy consumption (hence heat generation) of the front end electronics.